A multi-axis angular rate sensor is disclosed in commonly assigned U.S. Pat. No. 4,821,572. In this device, a plurality of pairs of accelerometers are mounted on a first and second frame member for rotation about a common axis. The first and second frame members are counter-rotated about the common axis, without transmitting a reaction force to a supporting base that is interposed between the frame members.
The drive mechanism used to counter-rotate the frame members, as disclosed in the above-referenced patent disclosure, comprises first and second C-shaped electromagnetic coils and associated pole pieces. Each electromagnetic coil and its associated pole piece are attached to different frame members so that when the coils are alternately and sequentially energized with an electric current, the frame members rotatably dither back and forth in opposite directions.
Several problems are associated with the prior drive mechanism used in the multi-axis rate sensor. Although the frame members only rotate a few degrees in each direction, the first and second electromagnetic coils are energized by current supplied through leads that are continually flexed as a result of the dither motion of the device. Eventually, even the most flexible conductors available may work-harden and break. Since the coils are attached adjacent the periphery of the frame members, the mass and rotational inertia of the frame members are substantially increased by the addition of the coils, although an important design goal for this device was to minimize these parameters.
Conventional direct current electromagnetic motors capable of developing the torque required to drive the multi-axis rate sensor are comparatively bulky. Their bulk is necessary to accommodate permanent magnets, ferrus metal flux linkage members, and pole pieces that focus the magnetic flux across air gaps in the motor. A rotor in a conventional motor typically includes armature windings that are energized through brushes, which produce radio frequency (RF) noise and are subject to wear. The mass of such a rotor and its inertia prevent it from quickly stopping and reversing direction. Accordingly, a conventional prior art DC motor is not usable to drive the multi-axis rate sensor described above. Conventional motors are neither sufficiently compact, nor do they include a rotor that is sufficiently low in mass and inertia to rapidly rotate back and forth through a small incremental angle. Since a conventional DC motor cannot easily be adapted to this application, it has been necessary to develop a new type of motor that meets these design criteria for driving the multi-axis rate sensor. The present invention was specifically developed for that purpose, but it is believed to have advantages over prior art motors that make it useful for many other applications. The advantages of the present invention over the prior art will be apparent from the attached drawings and the Description of the Preferred Embodiments that follows.